An image forming apparatus such as an inkjet printer discharges ink from a nozzle provided in an inkjet head. When moisture or volatile component is evaporated, a viscosity of the ink is increased and the ink is solidified. When the ink is solidified in the nozzle, a printing error may be caused.
In order to suppress the printing error caused by the solidified ink, for example, it is known that a cap is installed in the inkjet head. The cap covers the nozzle so as to suppress the moisture or the volatile component of the ink from being evaporated. Since increase of the viscosity of the ink is suppressed, the solidification of the ink is also suppressed.    Patent Document 1: Japanese Patent Application Laid-Open No. 2009-208372